Macrophages. dendritic cells and B-lymphocytes are sequentially recruited into the islets. serving as potent antigen presenting cells during the development of IDDM. This project will investigate the role of macrophages in the initiation of autoreactivity and islet cell damage. The efficient processing and presentation of autoantigens by macrophages is potentially a critical factor triggering pathogenesis. Engulfment of islet cell antigens by macrophages may lead to the generation of unique peptides which bind class II antigens and stimulate autoreactive T-cells. Studies of autoantigen presentation in humans have focussed on B- lymphocytes, yet little is known concerning the molecular steps which regulate antigen processing and presentation in macrophages. Antigen presenting cells may also regulate the progression of inflammatory responses, thus the influx of dendritic cells and later, B-lymphocytes into the pancreas may facilitate the presentation of new antigenic determinants. In support of this hypothesis, early T-lymphocyte responses are directed against a limited number of peptides derived from islet cell antigens followed by a broad spreading in T-cell reactivity with progression to IDDM. The overall goal of this project is to understand the molecular events which trigger immunological responses to islet cell antigens and the development of IDDM. For this purpose, the processing and presentation of an islet cell antigen, glutamic acid decarboxylase (GAD) will be studied in human macrophages, dendritic cells and B-lymphocytes. The initiation and amplification of autoimmune responses in IDDM, may be dependent upon the recruitment of specialized antigen presenting cells. Specifically, this proposal will test: 1) if macrophages, dendritic cells and B-lymphocytes differentially process the islet cell antigen GMD such that a distinct spectrum of antigenic peptides is produced within each of these APC; 2) if macrophages selectively display a unique series of GAD peptides bound to HLA class II antigens genetically linked with susceptibility to IDDM; 3) if macrophages preferentially activate early T-cell responses in prediabetes, while dendritic cells and B-lymphocytes are responsible for the spread of immunoreactivity with progression to IDDM. Insights into the relevant antigen presenting cells as well as the peptides derived from islet cell antigens, should prove useful in devising novel therapeutic interventions for IDDM.